I. Field of the Invention
The present invention relates to a method for fastening an accessory to a plastic fuel tank.
II. Description of Related Art
Fuel tanks on-board vehicles of various kinds must generally meet sealing and permeability standards in relation to the type of usage for which they are designed and the environmental requirements with which they must comply. At the present time, both in Europe and throughout the world, there is a substantial tightening of the requirements regarding the limitation of pollutant emissions into the atmosphere and into the environment in general.
To limit these emissions, care is taken in particular to position the accessories (ventilation lines, valves, baffles, stiffeners, etc.) inside the tank and/or the fill tube (see for example Application WO 2004/024487 in the name of the Applicant). However, when these elements are fastened to the tank after it has been moulded, it is generally necessary to make at least one opening in the tank so as to be able to introduce said elements into the tank and to fasten them thereto. Hence there may be potential sealing problems near this opening.
The Applicant has therefore developed a method of initially moulding a parison that includes at least one slot for introducing thereinto (and fixing thereto) accessories during the actual moulding of the tank and thus avoiding drilling openings (see for example Application EP 1 110 697). However, the Applicant has found that in such a method, as the tank cools down after moulding it undergoes considerable shrinkage (typically about 3%), whereas the parts that have been incorporated into it during moulding are heated only slightly and thus undergo appreciably less shrinkage, even when they are also made of plastic (given that there may be large differences in thermal expansion/contraction between polymers). This may introduce deformations (and therefore internal tensile stresses) that are unacceptable in the case of a component fastened to the tank at at least two points. Large deformations may also be observed in service, by the wall of the tank swelling on contact with fuels.
Application WO 2006/008608 (the content of which is introduced for reference in the present application) relates to a particular fastening mode (snap-riveting). A variant of this method addresses the problem of deformation (contraction, expansion), but solves it by one particular form of the snap-rivet and of the associated hole (the rivet is oblong and formed in an oblong hole that also permits a sliding movement of the accessory relative to the wall of the tank when it is fixed thereto). To prevent the formation of a bead of material in the oblong hole that would block the sliding movement, the snap-rivet must have a shape controlled by a suitable tool (counter-former) that is preferably cooled, but this complicates both the method and the tooling.